Area gradation processing is used to represent a gray-scale or halftone image on a display,, a printer, or a plotter capable of displaying with only two values, 0:OFF and 1:ON. This area gradation processing, widely known, refers to the processing based on a method for representing gray levels of an image by changing the ratio of ON dots to all dots in a relatively small unit area of an image. In general, an N*N matrix (where, “*” indicates a multiplication operator) produces N*N+1 gray levels. This is because the matrix gives unit areas composed of respectively N*N+1 different ON dots, from 0 to N*N. For example, an 8*8 matrix gives 65 gray levels. The larger value of N gives more gray levels but requires more memory capacity.
A 4*4 matrix or an 8*8 matrix is usually used for an image forming device such as a printer or a plotter that analyzes vector data. Recently, some image forming devices use a 128*128 matrix.
Regarding dot arrangement patterns in the matrix, dot distribution types such as a Bayer type and dot concentration types such as an eddy type and a mesh type are known. It is said that the dot distribution type is better in resolution and that dot concentration type is better in linear reproducibility of gradation.
FIG. 5 shows an 8*8 Bayer type dither matrix. Numeric values, 0 to 252 in increments of 4, are assigned to 64 cell positions, and each numeric value is used as a threshold for binarization or conversion from multi-level data to binary (or bi-level) data.
Normally, one vector is represented by the coordinates of the starting-end point and the terminating-end point, line width, end-point edge shape, gray level, and so on. Therefore, a dithering matrix cannot be applied directly to vector data unlike to image data. To represent the gray level of vector data, it is necessary to form a mask matrix having an ON/OFF dot pattern corresponding to a density (gray level) value of the vector based on the dither matrix as shown in FIG. 5.
FIG. 6 shows a mask matrix for density of 50%. In the case of the dither matrix shown in FIG. 5, the mask matrix is formed in the following manner. That is, if the density value of 100% is 255, then the density value of 50% is 127. Thus, the cells in the matrix shown in FIG. 5 whose values are 127 or smaller are set to ON with the cells whose values are larger than 127 set to OFF. In the example shown in FIG. 5, black (hatching) cells indicate ON and white cells indicate OFF.
FIG. 7 shows a mask matrix for density of 33%. In this case, because the density value is 85, the cells whose values ranges from 0 to 84 are set to ON with the other cells set to OFF. Similarly, FIG. 8 shows a mask matrix for density of 66%.
Referring to FIG. 22, how to represent halftone (shade) of a vector will be described. The figure to the left of the arrow in FIG. 22 indicates a vector represented by data such as starting-end and terminating-end points. The figure to the right of the arrow indicates the output result of the vector data for density of 50%. To convert vector data to halftone raster data in this way, the mask matrix described above is repetitively tiled (close to each other) beginning with the anchor corner (filling base point) as shown in FIG. 19 and, when vector data is to be converted to raster data, the raster pixels for which no gray level is considered (fully ON) are ANDed with the tiled ON/OFF dot pattern to create raster data.
FIG. 19 shows a mask matrix tiling pattern for density of lower than 2% as a typical low density. The example in the figure is to show whether or not data of a vector of one dot in line width generates ON dots when the density is lower than 2%. A mask matrix for density lower than 2% but not 0% is shown in FIG. 14. A pattern obtained by tiling this mask matrix is the mask matrix tiling pattern shown in FIG. 19. Line A shown in FIG. 19, one dot in width, is a horizontal line that lies on ON dots of the tiling pattern, with one ON dot generated every eight dots. However, lines B and C, each one dot in width, generate no ON dots at all and have their vector data fully lost because of their positions and inclination angles.
Recently, an increase in computer performance makes available a variety of graphics and CAD application software and diversifies color processing. Normally, when a print driver or a plotter driver creates vector data, the driver generates pen colors and density values and transfers the data to the printer or plotter. At this time, a low density, if set by the application, may cause vectors to be fully lost as described above.
It is an object of the present invention to provide a highly reliable image processing method and an image forming device that prevent vectors from being fully lost even in a halftone image of a relatively low density.
In an image forming device using an ink jet recording method, a plurality of ink-eject nozzles are usually arranged in the direction substantially perpendicular to the direction in which the recording head scans. Therefore, one carriage scan forms a band of an image area (band). The higher the scan speed is, the higher is the print speed. This is because data is usually printed based on the output pulses from a unit such as a linear scale sensor which is provided to detect the carriage position at a predetermined resolution (for example, 360 DPI) and because the frequency (head driving frequency or dot frequency) of the output pulses is determined according to the carriage scan speed.
An increase in print speed depends in part on whether or not the speed of the ink ejection from the ink jet head can keep up with an increase in print speed. If the head driving frequency is increased when the ink supply is insufficient, the print speed is increased but ink is not ejected properly with the result that some parts of the image cannot be printed. This significantly degrades the print quality.
To prevent this condition, there is a technology that reduces the head driving frequency (to the speed compatible with ink supply speed) only for a region within which the printing is performed and increases the head driving frequency only for non-print regions. However, to increase the print speed, it is preferable that the head driving speed is increased even in the print region.
It is another object of the present invention to provide an ink jet image forming device having a high-speed print mode that increases the head driving frequency even during printing in order to increase the print speed.
It is still another object of the present invention to provide a single-pass ink jet image forming device, wherein an interpreter in the ink jet image forming device thins out print dots, when forming fill patterns or thick-line shade patterns, to prevent ink dots from being ejected successively in the head scanning direction in order to allow ink to be sufficiently supplied even when the head driving speed is increased.
It is yet another object of the present invention to provide a single-pass ink jet image forming device capable of reducing the loss of thin lines in a thinning operation.